This invention relates to a method and apparatus for incorporating feature substances into a paper web and to a paper machine having such an apparatus.
It is known to incorporate feature substances into documents of value made of paper, in particular bank notes, as security features, for example luminescent particles fluorescing in a characteristic color under suitable excitation radiation such as UV light. Feature substances refer here in general to substances with certain physical properties whose presence and/or arrangement can be checked due to these properties by measurement technology, for example by suitable sensors. Such features are usually placed at defined positions in the paper as characters, patterns or lines.
It is known for example from DE-A-197 54 776 to spray colored patterns with sharp contours onto finished paper in linear form so as to produce graphic security features recognizable to the naked eye. Said security features are deposited on the surface of the paper and are therefore not only visible but also tangible. In particular when using luminescent substances whose color effects are only recognizable under certain excitation conditions, however, it is desirable that their place of incorporation is inconspicuous to the casual viewer and in particular to possible forgers.
UK-A-696 673 proposes for example injecting coloring pigments in a suspension liquid immiscible with water into the center of the sheet from a jet or nozzle during sheet formation to produce dotted lines or continuous pipes, for example of material fluorescent in UV light. However, since the fluorescent suspension spreads at least partially and uncontrollably in the not yet fully dipped paper material, the contours of such lines are blurred and the pigment concentration is uneven across the line width.
DE-C-497 037, in contrast, proposes applying, for example spraying, a suspension with fluorescent substances onto the fully dipped, still moist paper web in such a way that the paper structure itself does not undergo any appreciable change. However, spraying also leads to patterns whose contour acuity is difficult to control and whose feature concentration is inconstant across the surface of the pattern.
These disadvantages are partly overcome by the method described in UK-C 643 430 wherein an endless metal band with stencil-like gaps is moved together with the arising paper web and the colored feature substances are sprayed on diffusely so as to penetrate into the paper web in the area of the stencil-like gaps. However, this also fails to obtain a sufficiently homogeneous distribution of feature substances, as EP-A-0 659 935 criticizes.
EP-A-0 659 935 instead proposes dispersing feature substances not in suspension but in gas, so that agglomerates of feature particles readily break down and are present in the gas in a defined, homogeneous concentration, to then be sprayed onto the still wet paper web by a nozzle. This is said to achieve a homogeneous distribution in paper at the same time as relatively sharp contours even at low feature concentrations.
The disadvantage of this aerosol application of the particles is that only few feature substances are suitable for application in aerosol form since the pipes and nozzles are easily clogged. This applies in particular to fine-grained feature substances which tend to agglomerate. Furthermore, test results have shown relatively high fluctuations in concentration so that a high feature concentration is necessary for obtaining reliably measurable features.